In a new study, scientists looked at how the Golden State’s forests have responded to minuscule rainfall, high temperatures, and beetle infestations over the past four years. But instead of focusing on how many trees have died, Greg Asner, a global ecologist at the Carnegie Institution for Science, used a high-resolution mapping platform and laser-guided spectrographic imagery from airborne surveys to get a better understanding of how the entire forest was reacting to the drought.

“California relies on its forests for water provisioning and carbon storage, as well as timber products, tourism, and recreation, so they are tremendously important ecologically, economically, and culturally,” Asner, lead author of the study, said in a statement. “The drought put the forests in tremendous peril, a situation that may cause long-term changes in ecosystems that could impact animal habitats and biodiversity.”

The team found that up to 888 million large trees—those with a diameter of at least five inches at 4.5 feet above the ground—experienced a measurable loss of water in their canopies between 2011 and 2015. Of that group, as many as 58 million trees saw water loss levels deemed extremely threatening to long-term forest health. The team filtered out areas affected by fires between 2011 and 2015 to avoid skewing the results.

A tree’s canopy water content—the amount of water it contains in its foliage—is a good indicator of the tree’s overall health. When canopy water content is low, trees are more susceptible to fires and invasive beetles, and they also provide less shade and protection for plants and wildlife on the forest floor.

“We found massive areas of progressive canopy water stress that are geographically aligned with a growing population of observed dead trees,” the authors wrote in the study published in the Proceedings of the National Academy of Sciences. “There exists a pool of trees spread over millions of hectares of forest that may undergo sufficient canopy water content (CWC) loss to result in death. Based on rates of CWC change observed during the drought, this pool could increase into the hundreds of millions of trees.”

Asner said the airborne monitoring is giving state and federal entities better information to make decisions on how to mitigate forest losses due to drought.

“The Carnegie Airborne Observatory's research provides invaluable insight into the severity of drought impacts in California's iconic forests,” said Ashley Conrad-Saydah, deputy secretary for climate policy at the California Environmental Protection Agency, in a statement. “It will be important to bring their cutting-edge data and expertise to bear as the state seeks to address the effects of this epidemic of dying trees and aid in the recovery of our forests.”

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